Grounding system for tubular collectors in electrostatic precipitating apparatus

ABSTRACT

Tubular electrodes, generally of a polygonal configuration, and composed of nonconducting synthetic resin material, whose surfaces are rendered conductive by the deposition of a conductive--liquid film in electrostatic precipitators of the tube type, are grounded or otherwise connected to a terminal of the electrostatic power supply by a bibulous textile strip along the lower edges of the electrode adapted to collect conductive liquid.

United States Patent [1 1 Hardt Feb. 26, 1974 [54] -GR()UND!NG SYSTEM FOR TUBULAR 3,248,857 5/1966 Weindel et a1 1. 55/155 X COL CT IN ELECTROSTATIC 3,513,635 5/1970 Eishold et a1. 55/157 X PRECPITATING APPARATUS 3,577,705 5/1971 Sharlit 55/154 [75] Inventor: Lothar Hardt, Eschborn, Germany FOREIGN PATENTS OR APPLICATIONS 1, 69,413 91958 F .55 5 731 Ass1gnee: Metallgesellschaft A.G., Frankfurt 4 am Germany Primary Examiner-Dennis E. Talbert, Jr. Filed: June 8, 19 Attorney, Agent, or Firm-Karl F. Ross; Herbert 21 Appl. No.: 260,813 Dubno [30] Foreign Application Priority Data [57] ABSTRACT June 18, 1971 Germany P 21 30 074.3 Tubular electrodes, generally of a polygonal Configuration, and composed of nonconducting synthetic 52 us. c1 55/155, 55/156, 55/1310. 38 resin material, whose surfaces are rendered 51 Int. Cl. B03c 3/49 tive y the deposition of a conductive-liquid film in [58] Field of Search55/139, 154, 155, 157, DIG. 3s, electrostatic precipitators of the tube yp r 5 5 /15 grounded or otherwise connected to a terminal of the electrostatic power supply by a bibulous textile strip [56] R f e Ci d along the lower edges of the electrode adapted to collect conductive 2,592,508 4/1952 Ph l 55/DIG. 38 10 Claims, 7 Drawing Figures GROUNDING SYSTEM FOR TUBULAR COLLECTORS IN ELECTROSTATIC PRECIPITATING APPARATUS FIELD OF THE INVENTION The present invention relates to electrostatic precipitators and, more particularly, to electrostatic precipitators having tubular collector electrodes which are connected to one terminal of the power source, e.g. by grounding. More specifically, the invention deals with a system for grounding such electrodes.

BACKGROUND OF THE INVENTION In conventional systems for the removal of dust particles and other solids from air or a gas stream, electrostatic precipitators have found increasing significance because they are able to collect particles of the smallest size in an efficient manner with relatively high throughput.

Electrostatic precipitators generally comprise an array of collector electrodes having generally parallel surfaces extending in a gas-flow direction, and flanking corona-discharge electrodes, the collector electrodes and the corona electrodes being connected to opposite terminals of a high voltage power supply.

The corona discharge, along corona electrodes which may be simply wires, strips or the like, forms an ionized gas, the ions of which are absorbed or otherwise impart an electric charge to the particles which are drawn to the oppositely charged collector electrodes. The accumulated solids are then removed.

While a dry-operated electrostatic precipitator must provide rapping means for jolting the colector electrodesto cause them to shed the accumulation, wetoperating electrostatic precipitators may provide moisture or the like in the form of a liquid film or trickle which flows downwardly along the collector electrodes to carry away the accumulated solids.

The present improvement is directed to electrostatic precipitators of the latter type and particularly those which employ nonconductive tubular collector electrodes traversed by coaxial corona electrodes. The collector electrodes of the latter type may be composed of nonconductive synthetic resins and have surface conductivity imparted to them in the form of a conductiveliquid film. This film may be the washing liquid, e.g. which absorbs contaminants such as sulfur dioxide and sulfur trioxide to produce an inorganic electrolyte. Alternatively, the conductivity of the electrode may be supplied by dissolving salts therein before use. The liquid film may also derive in whole or in part from condensate from the gas stream.

It has been a common practice, in electrostatic precipitators of the wet type, to connect the film of liquid along the collector electrodes to one terminal of the power supply, generally also the ground, by immersing the lower end of the collector electrode in trough leading off the collected liquid accumulated from the descending film, cascade or trickle stream, and electrically connecting the contents of the trough to ground. This has the advantage that the liquid in the trough serves as the electrical connection to the electrodes, but is disadvantageous when polygonal tubes are employed or the collector electrodes are constituted as corrugated plates defining the tubes between them. With such electrodes, the troughs interfere with the corona-discharge electrodes and require excessive space. Attempts have been made to connect each of the tubular electrodes of an electrostatic precipitator by a wire or rod to ground, but these have also proved to be ineffective. Because of the corrosive environment, only platinum, lead and titanium wires have been found to be effective and these only under limited conditions. Other wires cannot be used because of the high acidity of the liquid when the system is employed for the dust collection in an industrial or other furnace gas purifier.

The use of graphite rods, glass fibers, ceramic leads, impregnated with graphite or serving as a conductor for the liquid, has also proved to be problematical, as there was never a guarantee of uniform current distribution over the area of the collector electrode.

It should be noted also that certain systems, which may have been successful for collector electrodes in the form of plates or individual tubes fail completely when the plates are brought together to define polygonal chambers or tubes between them and where nests of mutually contacting polygonal tubes are employed. It may also be observed that the requirements for efficient operation of a wet electrostatic precipitator are the rapid cooling of the gases, the efficient wetting of the surfaces of the collector electrodes, the rapid descent of any corrosive liquids and the structural stability of the collector electrode step. For this reason, collector electrodes of polygonal configuration have been found to be most desirable, especially when the polygonal tubes are in closely packed and preferably mutually contacting or contiguous relation.

OBJECTS OF THE INVENTION It is the principal object of the present invention, therefore, to provide an improved system for grounding polygonal collector electrodes of an electrostatic precipitator whereby the aforementioned disadvantages may be obviated.

It is also an object of this invention to provide a polygonal collector electrode structure which is connected to a terminal of a power supply, e.g. by grounding, the which is functionally reliable, efficient, mechanically stable and inexpensive or simple to make.

Still another object of the invention is to provide a wet-operating electrostatic precipitator in which the aforementioned disadvantages are obviated.

SUMMARY OF THE INVENTION These objects and others which will become apparent hereinafter are attained, in accordance with the present invention, by providing at the base and around at least part of the perimeter of each of a plurality of generally polygonal collector electrodes for an electrostatic precipitator, a bibulous strip adaped to collect the conductive liquid descending along the walls of the polygonal tubes and forming a conductor connecting the surface film of the tubes to one terminal of the power supply, generally the ground.

The bibulous strips are preferably composed of a synthetic resin material, which may be the same as that constituting the tubes, and may overlie the bottom edge or end of the tube, Furthermore, the collecting surface of each tube pre-ferably leads directly onto the bibulous strip as a result of the juxtaposition of the latter and the bottom edge of this surface. The strips thus extend at least partially, and preferably completely,

around theperiphery of the lower edge of the collector--electrode tube, partly in the tube edge and advantageously extend downwardly upon the same while leading off horizontally to connect the tubes with the ground.

The term ground is used herein in its electrical sense, although most frequently the ground potential will correspond to the potential of the surrounding earth. It is common practice with high voltage electric precipitators, to minimize the shock hazard to personnel, to electrically ground one terminal of the power source and to connect the collector electrode to the same ground potential.

The grounding arrangement according to the present invention has the advantage that the descending liquid film or trickle drains at the lower end of the collector tubes over the bibulous strips which retain the conductive liquid by absorption and capillary action within and between closely packed fibers or filaments from which the strip is constituted. Part of the draining liquid is also retained by the bibulous strips and is uniformly distributed throughout the strips and along any length of each strip which may extend from the stack of tubes to a terminal or the like. Even portions of the strip which may be directly below parts of the collecting surface free from liquid are uniformly wetted by the conductive liquid as a result of the capillarity of the bibulous material of the strip. Uniform conductivity along the strip is thereby assured and a uniform distribution and dissipation of electrical charge results.

According to another feature of the invention, the strip consists of a woven fabric or a nonwoven textile product. A woven fabric within the definition of the present invention comprises interweaved weft and warp filaments,each of which may be a monofilament or a yarn twisted from a number of monofilaments to increase the liquid-retention capacity of the strip. Where a thick portion or bead of the strip is desired,- directly below the liquid-shedding edge of the collecting electrode, the bead may be formed by turning over an edge of the woven band. The term nonwoven textile is used to designate any material constituted of monofilaments or yarn matted together, generally under heat and pressure, such that the termoplastic fibers are held in place. In practice it has been found that such nonwoven strips have a greater liquid capacity than woven strips of the same yarn content and composition. In the latter case the bead may be formed simply by bunching the nonwoven fibers.

Preferably the strips are comprised of synthetic resin filament and fibers which are resistant to corrosive action by the aggressive substances present in the liquid or the gases traversing the electrostatic precipitator and having a thermal resistance enabling the strips to operate without fusing at the operating temperatures of the precipitator. It is even possible to use many synthetic resins which are considered hydrophobic or nonwettable by water since the woven or nonwoven character of the fabric provides capillary channels along which the liquid can be distributed and is induced to collect. I may, moreover, treat the surfaces of the strips to increase conductivity or wettability.

Any conventional treatment associated with the synthetic resin which is employed, may be used for this purpose. For example, with polyethylene. polyamide or vinyl polymers constituting the synthetic resin strips, the strip may be exposed to electron beam treatment along the surface of its corona discharge treatment to improve the wettability. An additional treatment which may be employed is that of grafting acrylates or the like to the synthetic resin backbone underlying electrostatic fields, with corona discharge or in the presence of cztalysts to render the strips wettable. Where it is desired to increase the electrical conductivity of the strips beyond that which is attainable with the liquid employed, I partially or completely graphitize the filaments or fibers constituting the strip and/or embed graphite particles in the strip after it has been formed.

The nonwoven strips which have been found to be most desirable for the purpose of the present invention are those which have heretofore been used as wadding.

The strips may be simply clamped between adjoining surfaces of two or more collector electrodes, in which case they may have beads underlying the collector surfaces of the latter, or may be bonded, e.g. by adhesive or under heat and pressure, to either the collector surface or a noncollecting surface of the polygonal tubes. For this purpose each strip may comprise a web leading upwardly from the respective bead. When the tubes are formed from sheets of corrugated synthetic resin brought into juxtaposition, the strips are bonded to these walls, the adhesive or thermal welding of the strips to the polygonal collector electrodes not only provide reliable electrical conductivity between them but also ensure continuity between any surface film of liquid and the liquid collecting in the bibulous strip. The preferred synthetic resins for both the tube and the strip are polyvinylchlorides, polyesters or polytetrafluoroethylene.

Where the polygonal tubes are formed by walls, at least one strip is provided at the lower edge of each wall so that each polygonal tube is provided along its lower periphery with portions of two such strips. However, every other wall may be formed with a strip in which case each polygonal tube has a strip portion lying over half of its periphery.

Where at least one strip is provided at the lower end of the end wall, assembly of the walls results in a gtounding of the individual polygonal tubes along their interiors at least over half the periphery.

It should also be mentioned that the bibulous strips of the present invention have been found to be advantageous because the surface tension of the liquid is reduced as a conse-quence of the electrical charge produced by the electrostatic field. Thus even normally hydrophobic materials will be suitable as bibulous strips because of the reduced tendency of the water to roll of the strips.

DESCRIPTION OF THE DRAWING The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being madeto the accompanying drawing in which:

FIG. 1 is a bottom perspective view of a portion of a stack of collector electrodes for an electrostatic precipitators according to the present invention;

FIG. 2 is a view similar to FIG. I of another collector electrode wherein the tubes are formed by juxtaposed walls; an

FIGS. 3A through 3E are perspective sections through various arrangements of strips according to the invention.

SPECIFIC DESCRIPTION In FIG. 1, I have shown a portion of an electrostatic glass fibers, coaxially surround respective corona discharge electrodes 11 connected to one terminal of a high voltage power supply 12 the other terminal of which is grounded. The condensate collects upon the inner surface la of the electrodes 1 and descends with entrained dust particles for collection in a trough 13 by which the liquid is led from the system. In the present embodiment, the gases entrained contain sulfur oxides or like constituents which render the liquid conductive so that theinner surface la of each tube is coated with a conductive-liquid film. An electrostatic precipitator of this general type is described in commonly assigned US. Pat. No. 3,513,635.

The collector-electrode tubes are of hexagonal crosssection and are stacked in mutually contacting relationshipm to form a hneycomb structure. Each of the tubes 1 of the honeycomb is rendered conductive by the liquid as noted and is electrically connected to ground by a strip 2 which winds across the array below the lower edges of the tubes to lie along half of the periphery of each tube contacted by the strip. The strip 2 is composed of woven or nonwoven textile formed from synthetic resin fibers or filaments as previously described. The parallel rows of bibulous liquid collecting strips 2, which rapidly become saturated with the conductive liquid and thereafter shed the excess, are clamped be- .tween adjoining tubes of the array along their external surfaces but have depending portions 2a underlying the lower edges.

In FIG. 2, I show another honeycomb array of tubes formed by juxtaposed sheets 4 of corrugated configuration. In this case, the strips 2 of synthetic resin material are adhesively secured or thermally bonded to the lower edges of the wall 4 and extend around the lower edge of each of the polygonal tubes.

FIGS. 3A through 3E illustrate various arrangements of bibulous strips 2. For example, the strip 2 of FIG. 3A is a simple woven band of polyester fibers which may be partly graphitized and which is bonded at 2b to the external surface 1b of the associated tube or the wall 4 of the system of FIG. 2. An adhesive bond and a thermal weld are equivalent for this purpose. The lower edge of the tube is preferably beveled at 14 to lead the liquid film smoothly onto the portion of the strip 2 depending below the tube. Where the tube is not beveled, I may make use of two strips 15 and 16, together constituting the strip 2, having upper webs 15a and 16a securing the tube 1 on the wall 4 between them. In this case, the woven bibulous strips encase'the lower edge of the tube and a uniform transfer of liquid is provided by contact by one of the strips with the inner surface of the tube.

In FIG. 3C, two tubes 1 having beveled inner surfaces are shown to clamp a single bibulous strip 2 between them. This system operates similarly to that of FIG. 3A.

In FIGS. 3D and 3E nonwoven strips 5 of bibulous synthetic resin fiber are employed, the strips having webs 5a and fiber filled beads 5b, the latter immediately underlying the edge 10 of each tube 1. In the system of FIG. 3D, the web 5a is bonded to the external surface of the tube while the bead projects inwardly to underhand the inner surface. In the system of FIG. 3E, the web is clamped between two tubes so that the bead bulges inwardly with respect to each tube to underlie the respective inner surface.

I claim:

1. In an electrostatic precipitator having an array of generally tubular upright passages defined by liquidconducting surfaces, the improvement which comprises a grounding arrangement for electrically connecting said surfaces to a power source, said grounding arrangement comprising at least one bibulous strip of synthetic resin material extending at least partly around each of said passages and below said surface at the bottom of each passage for absorbing a conductive liquid.

2. The improvement defined in claim 1 wherein said passages are defined by individual tubes in contact with one another, said strips being received between the external surfaces of adjacent tubes and projecting therebeneath.

3. The improvement defined in claim 1 wherein said passages are defined between juxtaposed plates having corrugations forming said passages, said strip being provided along the bottom of at least one of said plates and being fixed thereto.

4. The improvement defined in claim 1 wherein said strip is composed of a woven textile material.

5. The improvement defined in claim 1 wherein said strip is composed of a nonwoven textile material.

6. The improvement defined in claim 1 wherein said passages are each formed by a tube wall having an outer surface, said strip being formed with a web lying along said outer surface and a bead underlying the surface of the respective passage.

7. The improvement defined in claim 1 wherein said electrostatic precipitator is formed with an array of said passages in hexogonal configuration, further comprising corona discharge electrodes extending centrally through certain passages, a high voltage power supply having one terminal connected to said corona discharge electrodes and another terminal grounded, and a conductor connecting said strip to ground.

8. The improvement defined in claim 7 wherein said passages are defined by individual hexagonal-section synthetic resin tubes.

9. The improvement defined in claim 7 wherein said passages are defined between corrugated sheets and are of hexagonal configuration.

10. The improvement defined in claim 7 wherein said strips are partially graphitized. 

1. In an electrostatic precipitator having an array of generally tubular upright passages defined by liquid-conducting surfaces, the improvement which comprises a grounding arrangement for electrically connecting said surfaces to a power source, said grounding arrangement comprising at least one bibulous strip of synthetic resin material extending at least partly around each of said passages and below said surface at the bottom of each passage for absorbing a conductive liquid.
 2. The improvement defined in claim 1 wherein said passages are defined by individual tubes in contact with one another, said strips being received between the external surfaces of adjacent tubes and projecting therebeneath.
 3. The improvement defined in claim 1 wherein said passages are defined between juxtaposed plates having corrugations forming said passages, said strip being provided along the bottom of at least one of said plates and being fixed thereto.
 4. The improvement defined in claim 1 wherein said strip is composed of a woven textile material.
 5. The improvement defined in claim 1 wherein said strip is composed of a nonwoven textile material.
 6. The improvement defined in claim 1 wherein said passages are each formed by a tube wall having an outer surface, said strip being formed with a web lying along said outer surface and a bead underlying the surface of the respective passage.
 7. The improvement defined in claim 1 wherein said electrostatic precipitator is formed with an array of said passages in hexogonal configuration, further comprising corona discharge electrodes extending centrally through certain passages, a high voltage power supply having one terminal Connected to said corona discharge electrodes and another terminal grounded, and a conductor connecting said strip to ground.
 8. The improvement defined in claim 7 wherein said passages are defined by individual hexagonal-section synthetic resin tubes.
 9. The improvement defined in claim 7 wherein said passages are defined between corrugated sheets and are of hexagonal configuration.
 10. The improvement defined in claim 7 wherein said strips are partially graphitized. 